The present invention relates to a method and an apparatus for detecting a spectrum spread signal in additional information signal which has been spectrum spread and superimposed on a main information signal such as, for example, a video signal.
Digital information recording apparatuses such as digital VTRs and MD (Minidisc) recording and playback apparatuses have become widespread, and further DVD (Digital Video Disc or Digital Versatile Disc) apparatuses each equipped with a recording function have also come on the market. In these digital information recording apparatuses, various additional information signals are allowed to be recorded in the wake of digital video signals, digital audio signals and further data for computers and the like, which are main information signals.
FIG. 1 is a view showing a structural example of a conventional recording apparatus adapted to add an additional information signal onto a digital information signal for recording. In the recording apparatus of the example of FIG. 1, a digital information signal Vi is supplied to a recording apparatus 10 through an input terminal 11. In this recording apparatus 10, an additional information signal is added onto the digital information signal Vi to be recorded on a recording medium 15.
In an addition unit 12 of the recording apparatus 10, an additional information signal from an additional information generating unit 13 is added to the digital information signal Vi. In this case, this additional information signal is a digital signal, and is added to a digital information signal Vi as a signal to be recorded in such an area as is regionally distinguished from the digital information signal, such as a header portion to be added to data of, for example, a digital information signal in units of blocks, and an area for other TOC (Table of Contents).
A digital information signal, to which an additional information signal from the addition unit 12 is added, is recorded on a recording medium 15 such as tape or a disc by a writing unit 14. In this case, if compression encoding is required for the digital information signal as described above, the compression encoding process is performed in the writing unit 14.
As described above, in the case of a conventional recording system for an additional information signal, the additional information signal is not directly superimposed on a digital information signal, but is arranged to be added to an indirect portion such as a header portion. For this reason, the additional information signal is comparatively easily removed by filtering or tampering, thus possibly making it impossible to detect through a necessary additional information signal by a recording apparatus or a playback apparatus. Particularly, if control information to prevent unauthorized duplication, copyright information or the like is added as an additional information signal, such loss of the additional information signal will bring about such a state that the initial object cannot be achieved.
Also, in the case of addition of an additional information signal onto such an indirect portion as described above, the additional information signal will be lost when the digital information signal is converted into an analog signal because only the main information signal can be obtained. Even if a duplication-preventive control signal is superimposed as an additional information signal, and such a measure as to prevent unauthorized duplication of a digital information signal is taken, the duplication-preventive measure is not effective any longer when converted into an analog signal.
As a superimposition system for an additional information signal capable of solving the problem on the lost additional information signal and the problem when the additional information signal is converted into an analog signal as described above, the present applicant previously proposed a system for spectrum spreading an additional information signal such as the duplication-preventive control signal, and superimposing this additional information signal thus spectrum spread on a video signal to digital-record or analog-record the video signal (U.S. patent application Ser. No. 08/75510).
In this system, a code (hereinafter, referred to as PN code) of PN (Pseudorandom Noise) series, which is used as a spread code, is caused to be generated at a sufficiently high rate, and this code is spectrum spread by applying it to the additional information signal, whereby an additional information signal such as a narrow band duplication-preventive control signal at a high level is caused to be converted into a wide band signal at an infinitesimal level which does not affect the video signaloraudio signal. Thus, this additional information signal thus spectrum spread, that is, the spectrum spread signal is superimposed on the analog video signal to record on a recording medium. In this case, a video signal to be recorded on the recording medium may be either analog or digital.
In this system, the additional information signal such as the duplication-preventive control signal is spectrum spread and superimposed on a video signal as a wide band signal at an infinitesimal level, and therefore, it is difficult, for, for example, a person who intends to duplicate illegally to remove the duplication-preventive control signal superimposed from the video signal.
On the other hand, it is possible to utilize by detecting an additional information signal such as the duplication-preventive control signal superimposed by an inverse spectrum spread. Therefore, it is possible to reliably supply the duplication-preventive control signal together with, for example, the video signal to a recording apparatus side, and on the side of this recording apparatus, it is also possible to detect this duplication-preventive control signal for reliably performing duplication control corresponding to the duplication-preventive control signal detected.
In the case of spectrum spreading an additional information signal to superimpose it on a main information signal such as a video signal, the spectrum spread signal is not removed from the main information signal at the playback of the main information signal, and therefore, it is necessary to superimpose the spectrum spread signal at an infinitesimal level, at which the playback output of the main information signal is not affected.
When, however, the spectrum spread signal is superimposed on the main information signal at such an infinitesimal level, it is important that the spectrum spread signal can be reliably detected from the main information signal.
When the spectrum spread signal is detected from a signal Si in which the spectrum spread signal is superimposed on the main information signal, the evaluation function xcfx86 at the time of the inverse spectrum spread is given by the following equation:                                                         φ              =                              ∑                                  Si                  ·                  pi                                                                                                        =                              ∑                                                      (                                          Vi                      +                                              ki                        ·                        pi                                                              )                                    ⁢                  pi                                                                                                        =                                                ∑                                      Vi                    ·                    pi                                                  +                                  ∑                                      ki                    ·                    pi                    ·                    pi                                                                                                          (        1        )            
where Vi indicates the main information signal such as a video signal, pi, a spread code such as PN code, and ki, the coefficient respectively.
In this equation (1), the first term represents a correlation between the main information signal and the spread signal, and the second term represents a correlation between the spectrum spread signal and the spread code. From this equation (1), it can be seen that the spectrum spread signal could be easily detected if there is no correlation between the main information signal and the spread signal.
Actually, however, the main information signal such as a video signal is correlated with the spread signal. More specifically, if it was a flat signal which has no variations, the main information signal would not be correlated with the spread code, but since a general main information signal varies depending on the content of the information, it cannot be said that it is not correlated with the PN code.
For this reason, it is likely to be difficult to detect the spectrum spread signal obtained by spectrum spreading the additional information signal because of this correlation between the main information signal and the spread signal depending on the content of the information of the main information signal.
In light of the above-described problems, it is an object of the present invention to provide a method and an apparatus capable of reliably detecting the spectrum spread signal by lowering or suppressing the correlation between the main information signal and the spread signal.
The present invention concerns a method and an apparatus for detecting an additional information signal from an input signal produced by superimposing a spectrum spread, which has been generated by spectrum spreading an additional information signal, on a main information signal, and is characterized in that a change in level of the main information signal contained in the input signal is restrained to generate a level restraining signal, and the level restraining signal is inverse spectrum spread to detect the additional information signal from the input signal. The change in level of the main information signal is restrained by restraining the gain of the input signal by a gain control circuit or slicing the input signal at a predetermined level by a slicing circuit.
Thus, the main information signal in the input signal becomes less correlated with the spread code because it becomes a signal which hardly varies by gain control, or its major portion is removed by slicing. Therefore, it becomes easier to detect the additional information signal spectrum spread.